1. Field of the Invention
The present invention relates to a linear guideway, and more particularly to an anti-dust structure for a sliding block of a linear guideway, which can use the flexible anti-dust strip to scrape off the grease and dust accumulated on the top surface of the rail at any time, preventing the dust and grease entering the upper rolling element groove via the top surface of the rail, and thus obtaining a better anti-dust function and ensuring the service life of the linear guideway.
2. Description of the Prior Art
A linear guideway is normally provided in its upper inner surface of the sliding block with an anti-dust structure that is located correspondingly to the upper ball groove for preventing dust or grease from accumulating on the top surface of the rail. The anti-dust structures commonly seen include the following two types:
A first conventional anti-dust structure disclosed by U.S. Pat. No. 4,918,846 is shown in FIG. 1, a U-shaped anti-dust plate 10 is fixed to the end of the sliding  block by screws. The anti-dust plate 10 is made of spring steel and formed by punch forming, and then the lip portion 11 is fixed at either side of the anti-dust plate 10 (can be fixed by bonding method). The method of fixing the plate 10 on the sliding block includes two types: first, drilling a plurality of threaded holes in the sliding block for engaging with the screws of the anti-dust plate 10; second, an oil nozzle structure is formed in the outer end of the sliding block for fixing the anti-dust plate 10. The lip portion 11 is adapted to scrape dust and grease off the top surface of the rail. However, this conventional product is impractical because of the following reasons:
First, in addition to the complicated assembly process, it should use a plurality of screws to fix the anti-dust plate 10, and some anti-dust plates 10 should be fixed by using the oil nozzle structure. Obviously, it is uneconomic since the resultant production cost is pretty high.
Second, the production process is complicated, the producer has to punch the anti-dust plate 10, and then fix the lip portion 11 on it, and the sliding block also needs to be drilled.
Third, the lip portion 11 is easily broken off after bonding, it is impossible to use soft flexible material (soft flexible material is more difficult to be positioned by bonding), therefore, the dust accumulated inside the lip portion 11 still can enter the upper ball groove easily.
Fourth, the lip portion 11 is bonded to the anti-dust plate 10, the lip portion 11 and the anti-dust plate 10 must be replaced together once one of them is broken, and the replacement work requires the user to unscrew the screws with tools.  Therefore, the maintenance of the lip portion 11 is not easy.
To solve the abovementioned problems, another conventional anti-dust structure was developed, which is disclosed by U.S. Pat. No. 5,871,282 as shown in FIG. 2.
This anti-dust structure is a metal plate 13 directly mounted on the top inner surface of the sliding block 12 by screws 16, and a pair of lip portions 14 is integrally formed with the plate 13 during injection molding process. With the injection molding process, the lip portions 14 can be made of flexible material. For obtaining a better positioning effect, the lip portions 14 must be formed by two steps: the harder portion is firstly formed on the plate 13, then the softer layer with flexibility for contacting the top surface of the rail 15 is formed on the harder portion. Although the second conventional anti-dust structure has been improved, it is still not very practical because of the following reasons:
First, it still needs to use the screws 16 to fix the metal plate 13 and the lip portions 14, the resultant production cost is high.
Second, the production process is still complicated despite the assembly is simple, because the lip portions 14 should be formed by two steps, which involves two materials and many processes.
Third, the pair of lip portions 14 is formed by injection molding, it can't be required, and once it is broken, the whole anti-dust system must be replaced, and the replacement work requires the user to unscrew the screws with tools. Therefore, the maintenance of the lip portion 11 is not easy.
To solve the aforementioned problems, the inventor of this invention, on the  basis of the accumulated experience and skills associated with the linear transmission field, develops a simple structured, low cost anti-dust structure with improved anti-dust effect used on a sliding block of a linear guideway.